Nowadays, oil fields, which are found deep in the ground and have never been considered to date, and oil fields and gas fields in a severe corrosive environment, which is called a “sour” environment in which hydrogen sulfide or the like is present and so forth are being actively developed from the viewpoint of a sharp rise in the price of crude oil and the depletion of petroleum resources which is anticipated in the near future. These oil and gas fields are generally found very deep in the ground and in a severely corrosive environment in which the temperature of the atmosphere is high and CO2, Cl−, and H2S are present. A steel pipe for an oil well in this kind of environment is required to have not only high strength but also excellent corrosion resistance (resistance to sulfide stress cracking and resistance to carbon dioxide corrosion).
Hitherto, 13% Cr martensitic stainless steel pipes have been widely used as oil country tubular goods to be used for production in an oil and a gas field in an environment in which carbon dioxide CO2, chlorine ions Cl−, and so froth are present. Moreover, nowadays, modified 13Cr martensitic stainless steel, which has a chemical composition containing less C and more Ni and Mo than conventional 13Cr martensitic stainless steel, is increasingly being used.
For example, Japanese Unexamined Patent Application Publication No. 10-1755 discloses modified martensitic stainless steel (pipe) in which the corrosion resistance of 13% Cr martensitic stainless steel (pipe) is improved. The stainless steel (pipe) according to Japanese Unexamined Patent Application Publication No. 10-1755 is martensitic stainless steel with excellent corrosion resistance and resistance to sulfide stress corrosion cracking, the steel having a chemical composition containing 10% to 15% of Cr, in which C content is limited to 0.005% to 0.05%, Ni content is 4.0% or more, Cu content is 0.5% to 3%, and Mo content is 1.0% to 3.0%, while Nieq is adjusted to be −10 or more, and a microstructure including a tempered martensite phase, a martensite phase, and a retained austenite phase, in which the sum of the phase fractions of a tempered martensite phase and a martensite phase is 60% to 90%. It is disclosed that corrosion resistance and resistance to sulfide stress corrosion cracking in a wet carbon dioxide environment and a wet hydrogen sulfide environment are increased using this steel.
In addition, nowadays, oil wells in a corrosive environment at a higher temperature (as high as 200° C.) are being developed. However, there is a problem in that required corrosion resistance, which is satisfactory in this corrosive environment at a high temperature, cannot be stably achieved by the technology according to Japanese Unexamined Patent Application Publication No. 10-1755.
Therefore, a pipe with excellent corrosion resistance and resistance to sulfide stress corrosion cracking for an oil well, which can be used in a corrosive environment at such a high temperature, is required, and various kinds of martensitic stainless steel pipes have been proposed.
For example, Japanese Unexamined Patent Application Publication No. 2005-336595 discloses a high strength stainless steel pipe with excellent corrosion resistance, the steel having a chemical composition containing C: 0.005% to 0.05%, Si: 0.05% to 0.5%, Mn: 0.2% to 1.8%, Cr: 15.5% to 18%, Ni: 1.5% to 5%, Mo: 1% to 3.5%, V: 0.02% to 0.2%, N: 0.01% to 0.15%, and O: 0.006% or less, while a specified relational expression is satisfied by Cr, Ni, Mo, Cu, and C, while a specified relational expression is satisfied by Cr, Mo, Si, C, Mn, Ni, Cu, and N, and a microstructure including a martensite phase as a base phase and 10% to 60%, in terms of volume fraction, of a ferrite phase, or further, 30% or less, in terms of volume fraction, of a retained austenite phase. A stainless steel pipe for an oil well having high strength and toughness, which has satisfactory corrosion resistance even in a severe corrosive environment in which CO2 and Cl− are present and the temperature is as high as 230° C., can be stably manufactured using that steel.
In addition, Japanese Unexamined Patent Application Publication No. 2008-81793 discloses a high strength stainless steel pipe for an oil well having high toughness and excellent corrosion resistance. The steel pipe according to Japanese Unexamined Patent Application Publication No. 2008-81793 is a steel pipe, the steel pipe having a chemical composition containing, by mass %, C: 0.04% or less, Si: 0.50% or less, Mn: 0.20% to 1.80%, Cr: 15.5% to 17.5%, Ni: 2.5% to 5.5%, V: 0.20% or less, Mo: 1.5% to 3.5%, W: 0.50% to 3.0%, Al: 0.05% or less, N: 0.15% or less, and O: 0.006% or less, while a specified relational expression is satisfied by Cr, Mo, W, and C, while a specified relational expression is satisfied by Cr, Mo, W, Si, C, Mn, Cu, Ni, and N, while a specified relational expression is satisfied by Mo and W, and a microstructure including a martensite phase as a base phase and 10% to 50%, in terms of volume fraction, of a ferrite phase. A high strength stainless steel pipe for an oil well, which has satisfactory corrosion resistance even in a severe corrosive environment at a high temperature in which CO2, Cl−, and H2S are present, can be stably manufactured using that steel.
In addition, International Publication No. WO 2010/050519 discloses a high strength stainless steel pipe with excellent resistance to sulfide stress cracking and resistance to high temperature carbon dioxide corrosion. The steel pipe according to International Publication No. WO 2010/050519 is a steel pipe, the steel pipe having a chemical composition containing, by mass %, C: 0.05% or less, Si: 1.0% or less, Cr: more than 16% and 18% or less, Mo: more than 2% and 3% or less, Cu: 1% to 3.5%, Ni: 3% or more and less than 5%, Al: 0.001% to 0.1%, Mn: 1% or less, and N: 0.05% or less, while a specified relational expression is satisfied by Mn and N, and a microstructure including a martensite phase as a main phase, 10% to 40%, in terms of volume fraction, of a ferrite phase and 10% or less, in terms of volume fraction, of a retained γ phase. A high strength stainless steel pipe, which has satisfactory corrosion resistance even in a carbon dioxide environment at a temperature of as high as 200° C., and which has satisfactory resistance to sulfide stress cracking even in an atmosphere gas at a lowered temperature, can be manufactured using that steel.
In addition, International Publication No. WO 2010/134498 discloses a stainless steel for an oil well, the steel having a chemical composition containing, by mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.01% to 0.5%, P: 0.04% or less, S: 0.01% or less, Cr: more than 16.0% to 18.0%, Ni: more than 4.0% to 5.6%, Mo: 1.6% to 4.0%, Cu: 1.5% to 3.0%, Al: 0.001% to 0.10%, and N: 0.050% or less, while a specified relational expression is satisfied by Cr, Cu, Ni, and Mo, while a specified relational expression is satisfied by (C+N), Mn, Ni, Cu, and (Cr+Mo), a microstructure including a martensite phase and 10% to 40%, in terms of volume fraction, of a ferrite phase, while the ferrite phase has a length of 50 μm in the thickness direction from the surface of the steel and intersects at a ratio of more than 85% with virtual line segments placed in a line at intervals of 10 μm in a range of 200 μm, and a yield strength of 758 MPa or more. A stainless steel for an oil well, which has excellent corrosion resistance in an environment at a high temperature, and which has excellent resistance to SSC at room temperature, can be manufactured using that steel.
In the methods according to Japanese Unexamined Patent Application Publication No. 10-1755, Japanese Unexamined Patent Application Publication No. 2005-336595, Japanese Unexamined Patent Application Publication No. 2008-81793, International Publication No. WO 2010/050519 and International Publication No. WO 2010/134498, corrosion resistance is improved by setting Cr content to be more than 15 mass %. However, there is a problem in that an increase in the content of Cr, which is an expensive alloy element, causes a sharp rise in cost, which results in economic disadvantage.
It could therefore be helpful to provide a high strength stainless steel seamless pipe for an oil well, the pipe having excellent corrosion resistance (resistance to carbon dioxide corrosion) in a severe corrosive environment in which CO2 and Cl− are present and the temperature is as high as 200° C. and excellent corrosion resistance (resistance to sulfide stress cracking) in an environment in which H2S is present without an increase in Cr content and with a chemical composition having a comparatively low Cr content of about 15 mass % and a method for manufacturing the pipe. “High strength” shall refer to the case where the yield strength of the steel is 110 ksi (758 MPa) or more.